Unlike machines which fine spherical lenses, in machines used to grind, polish, and fine toric lenses, the lap is not rotated. It is given an irregular "break-up" motion the purpose of which is to constantly bring fresh points on the polishing elements into contact with each point on the work piece, thereby ensuring that the work piece will have a totally smooth surface.
Conventional laps used to grind, polish, and fine toric lenses comprise a body, means for translating the body in a "breakup" pattern during polishing of the work pieces, and a plurality of grinding, polishing, or fining elements (hereinafter referred to generically as "polishing elements") mounted on the working surface of the body such that the working surface of the polishing elements define a convex polishing surface. The body is typically made of aluminum, the means for translating the body is typically a rectangular stub shaft sized to be gripped by appropriate means on the grinding, polishing, or fining machine, and the polishing elements are typically aluminum pads to the working surfaces of which a plurality of diamond pellets are secured. The pads may be removable so that they may be individually replaced when the diamond pellets on each pad have worn to an unacceptable degree.
Conventional laps suffer from a serious flaw. The polishing elements are mounted in fixed positions on the bodies of the laps. As shown in schematic (and highly exaggerated) form in FIG. 1, the effective diameter of the working surface of the lap is the distance D from the radially outer point of each of the radially outermost diamond pellets to the radially outer point of the diametrically opposite outermost diamond pellet. However, as the diamond pellets wear down, the distance D decreases to the distance D'. (Of course, in actual practice both the wear on the pellets and the difference between D and D' are on the order of a few millimeters.) The "breakup" motion of the lap is designed to ensure that each point on the workpiece is traversed by a point on the polishing elements the same number of times, but the absence of polishing surface between the worn-down diameter D' and the original diameter D means that some points on the workpieces will not be traversed as often as others. This in turn causes imperfections in the finished product.